1. Field of the Invention
The present invention relates to an apparatus for reproducing a digital signal recorded on a recording medium.
2. Description of the Related Art
FIG. 1 illustrates a configuration of clock generation means which has been used in an optical disc player such as a CD (compact disc) player. An optical disc 2 is driven by a spindle motor 3 for rotation. Information stored on the optical disc 2 is read by a pick-up 4. An information signal reproduced from the pick-up 4 is amplified by an amplifier 5 and input to a binary slice circuit 6 which compares the read signal with a predetermined value to convert it into a binary pulse.
The binary pulse is compared with a reproduction clock generated by a VCO (voltage-controlled oscillator) 10 in a phase error detection circuit 8, and a resultant phase error is represented by a PWM (pulse width modulation) signal which is smoothed by an LPF (low-pass filter) 9 to provide an input voltage to the VCO 10. The VCO 10 reproduces a reproduction clock corresponding to the input voltage, and the output of the binary slice circuit 6 is sampled by a sampling circuit 7 to obtain a binary data output 11.
For example, in the case that a disc played by a CD-R (CD-Recordable) player is not only a CD-R but also a CD, there is a difference between their reproduced signals in the waveform at rising edges. This is attributable to a difference between their pit shapes resulting from a difference between the wavelength of a laser used for recording the CD and the wavelength used in recording the CD-R.
Since a phase error signal indicative of a phase error is generated by detecting signals in the neighborhood of edges of the signal waveforms, if there is a difference between the signal waveforms in the rise of edges, different phase error signals can be generated for identical phase errors.
Further, phase error signals can be adversely affected by noises when the gradient at edges of the signal waveforms is gentle. Therefore, a servo loop optimized for one of those discs will not act as the optimum loop for the other disc. Thus, the speed at which the reproduction clock is pulled into the optimum clock depends on disc types.
To solve this problem, a loop gain switching circuit as shown in FIG. 3 is used. Referring to FIG. 3, in the loop gain switching circuit, a phase error signal supplied to an input 902 is issued from an output 903 after being subjected to appropriate gain processing which is performed by switching a variable gain filter formed by the gain of an operational amplifier 901, a capacitance C1, and resistors R1, R3 or resistors R2, R4 with switches S1 and S2.
The speed at which the reproduction clock reaches the optimum clock may be adjusted by switching the gain of a loop filter depending on the disc by providing an LPF portion with gain switching means S1 and S2 as in the loop gain switching circuit shown in FIG. 3. However, analog switches like the switches S1 and S2 make shock and delay at the time of switching, which can cause a fluctuation in an input voltage to a VCO, thereby adversely affecting the reproduced clock.
Further, some discs have a recording area divided into information areas and servo areas as shown in FIG. 2. In such a case, a fixed pattern as indicated by As is recorded in each servo area, and the response speed must be increased in this section to introduce a lock state of a PLL (phase locked loop) quickly. In the information area indicated by Ad, it is necessary to achieve a slower response to prevent noises and the like from making the clock unstable.
In a continuous control system configuration to achieve this, as shown in FIG. 3, the loop gain is switched by an LPF such that the clock is quickly introduced into the servo area through expansion of the band of the LPF and such that the band is narrowed for the information area to suppress influence of noises and drop-outs if any.
However, a problem has arisen in that loop gain switching using analog switches or the like suffers from shock and delay at the time of switching, and this can cause a fluctuation in an input voltage to a VCO which adversely affects the reproduced clock and makes it difficult to perform quick access.
In addition, loop gain switching using an LPF may fail to provide a desired loop gain because of errors in each of elements such as R1 through R4 shown in FIG. 3. Thus, the loop gain switching at an LPF portion is susceptible to errors, although easy to implement. Loop gain switching can not be easily performed in a phase error detection portion because it outputs a PWM signal. Further, variations between disc layers and between disc types have also resulted in difficulties for continuous control systems in enabling quick access.
Furthermore, when a drop-out is caused by a scratch, fingerprint or the like, the resultant reproduced read signal is different from the correct signal recorded on the disc. Analog signal processing in an attempt to eliminate a phase error at the time of such a drop-out will fail because an analog switch or the like can not accurately eliminate it due to switching noises and the like. Thus, the resultant clock will not be accurate.